1. Field of the Invention
The present invention generally relates to a control apparatus for an internal combustion engine equipped with a fuel injection cut-off function which is triggered, for example, whenever a traction control function of the origins becomes active. More particularly, the invention is concerned with an ignition control apparatus for controlling a demand voltage of a spark plug to thereby prevent generation of high-level noise and protect circuit components of the ignition system from being damaged due to rises in the spark plug demand voltage.
2. Description of the Related Art
In general, in internal combustion engines (hereinafter also referred to simply as the engine) for automobiles or the like, it is required to control optimally the fuel injection and the ignition timing on the basis of the running or operation state of the engine. To this end, a microcomputer-based control apparatus is employed which detects a reference angular position of the crank shaft for each of the engine cylinders. This information is used to control the amount of fuel injected and the ignition timing relative to the reference angular position by using a timer in accordance with the relevant quantities calculated or arithmetically determined on the basis of the engine operation state.
Related to this, it is noted that modern automobiles or motor vehicles are increasingly equipped with a traction control capability for suppressing the engine output torque, with a view to preventing overrun, slippage on a frozen road or similar unwanted events. The torque control may be realized by stopping supply of the ignition signal to the igniter for the cylinder under control. However, in order to prevent fuel wastage as well as discharge of unburned fuel mixture into the atmosphere, it is preferred to adopt a torque control which is based on the cut-off or interruption of the fuel injection.
FIG. 7 is a block diagram showing a conventional control apparatus designed, for example, for a four-cylinder internal combustion engine in which the fuel supply to the engine is realized through fuel injection and the ignition control is realized by distributing a high voltage to spark plugs of the individual cylinders, respectively. Referring to the figure, an angular position signal generating means 1 is provided in association with a rotatable shaft of the engine, such as a crank shaft, cam shaft or the like. The generating means 1 generates an angular position signal T at every predetermined reference angular position of the crank shaft as the engine operates. To this end, the angular position signal generating means 1 may be constituted by an electromagnetic pick-up device disposed in opposition to a disk mounted on the crank shaft or cam shaft for rotation therewith and having a projection formed in the periphery of the disk, which projection passes by the electromagnetic pick-up device as the crank or cam shaft rotates. Alternately, the angular position signal generating means 1 may be implemented in the form of a photoarray disposed in opposition to slits formed in the disk mentioned above. In any case, the angular position signal T contains the reference position information for the angular positions of the crank shaft as well as the cylinder identification information.
For detecting the engine operation states, there are provided a variety of sensors denoted representatively in FIG. 7 by reference numeral 2. These sensors 2 detect engine operation states such as engine load, temperature etc. The detection signals outputted by these sensors 2, will hereinafter be referred to as the engine operation state signal and designated generally by a reference character D.
The angular position signal T and the engine operation state signal D are supplied to a control means 4 which is constituted by a microcomputer for controlling the operation of the engine on the basis of these signals T and D. More specifically, the control means 4 detects or identifies the reference positions for the individual cylinders from the angular position signal T and arithmetically determines or calculates the fuel injection quantity and the ignition timing on the basis of the engine operation state D to thereby output control signals J and Q for the fuel injection and the ignition timing, respectively.
Connected to the outputs of the control means 4 are fuel injectors 5 for injecting the fuel mixture into the associated cylinders, respectively, in response to the fuel injection signal J and an ignition coil 6 driven by the ignition timing signal Q. A distributor 7 is connected to a secondary winding of the ignition coil 6 and has output terminals connected to spark plugs 8 of the individual cylinders, respectively. When electrical conduction through a primary winding of the ignition coil 6 is interrupted in response to the ignition timing signal Q, a high voltage is induced in the secondary winding of the ignition coil 6 and applied to the spark plug 8 of the associated cylinder for firing the fuel mixture therein through the electric discharge of the spark plug 8.
The control means 4 includes input interfaces 41 and 42 for fetching the angular position signal T and the engine operation state signal D, respectively. It further includes a fuel control unit 43 for arithmetically determining or calculating the fuel injection quantity for each cylinder on the basis of the angular position signal T and the engine operation state signal D, and an ignition control unit 44 for calculating the ignition timing for each cylinder, again on the basis of the angular position signal T and the engine operation state signal D. The control means 4 also includes an output interface 45 for applying the fuel injection signal J indicative of the fuel injection quantity as calculated to the fuel injector 5, and an output interface 46 for applying the ignition timing signal Q corresponding to the ignition timing as calculated to the ignition coil 6.
Next, operation of the conventional engine control apparatus will be described by reference to FIG. 7.
As the engine rotates the angular position signal generating means 1 generates the angular position signal T indicative of the reference position. This signal T is then inputted to the fuel control unit 43 and the ignition control unit 44 incorporated in the control means 4 via the input interface 41. The various sensors 2 detect the engine operation states, whereby the engine operation state signal D is inputted to the ignition control unit 44 of the control means 4 via the input interface 42.
The fuel control unit 43 detects the reference position for each cylinder on the basis of the angular position signal T and arithmetically determines the fuel injection quantity as well as the fuel injection timing on the basis of the engine operation state signal D to thereby generate the fuel injection signal J corresponding to the calculated fuel injection quantity. Signal J is applied to the fuel injector 5 via the output interface 45. On the other hand, the ignition control unit 44 detects the reference position for each cylinder from the angular position signal T and calculates the ignition timing conforming to the engine operation state indicated by the signal D to thereby generate the ignition timing signal Q indicative of the calculated ignition timing, which signal Q is then applied to the ignition coil 6 via the output interface 46.
At this point in time, a timer control starts from a reference position determined on the basis of the angular point signal T. More specifically, the fuel injectors 5 are sequentially driven, whereby the fuel mixture is injected to the respective cylinders. Further, interruptions of the electrical conduction through the ignition coil 6 bring about electric discharges sequentially between a rotating center electrode and stationary peripheral electrodes of the distributor 7, which results in generation of sparks in the spark plugs 8 in a sequential manner, whereby the individual cylinders under control are fired correspondingly.
However, when the traction control mentioned previously is triggered, the fuel injection signal J to the fuel injector 5 associated with the cylinder for which the traction control is to be effected is inhibited, whereby the fuel supply to that fuel injector 5 is cut off. On the other hand, the ignition control unit 44 continues to generate the ignition timing signal Q. In conjunction with this, it has been observed that although the demand voltage of the spark plug 8 (i.e., the voltage required for the electric discharge to take place in the spark plug) in the normal fuel injection mode lies within a range of 10 kV to 20 kV, the demand voltage may rise to a range of 20 kV to 30 kV when the fuel supply to the cylinder associated with the spark plug 8 is cut off. The reason why the spark plug demand voltage increases when the fuel injection is cut off may be explained as follows. When the cylinder continues to remain in the state where the fuel injection is interrupted, temperature within the cylinder decreases, as a result of which emission of thermions from the cathode electrode of the spark plug 8 decreases. At the same time, the cylinder pressure rises abnormally during the compression stroke due to increase in the air density or concentration.
In particular, in the case where the ignition timing is set in the vicinity of the top dead center (TDC), the increase in cylinder pressure during the compression stroke as well as that of the spark plug demand voltage can no longer be neglected.
Parenthetically, it should be added that fuel injection is also interrupted in an engine deceleration region, where the throttle valve is fully closed, because no output torque is demanded when the throttle valve is fully closed.
Moreover, such a rise of in the cylinder pressure is also observed when an abnormality occurs in the pressure control for a supercharger employed in a turbo-engine. In general, in turbo-engines, the intake air quantity is increased by using the supercharger in order to make available the output torque which is in excess of the stroke volume or cylinder capacity, wherein a fail-safe mechanism is provided for preventing the cylinder pressure from increasing beyond an upper limit value. Accordingly, when failure or abnormality occurs in the fail-safe mechanism, there exists a high probability of the cylinder pressure increasing abnormally.
When the voltage demand of the ignition plug 8 becomes high for the reasons mentioned above, serious problems arise, such as generation of high level electric noise injury or damage to the distributor 7, the spark plug 8 and/or other circuit components due to leakage of the abnormally high voltage. To protect the circuit components against damage from the voltage leakage, it is conceivable to increase the voltage withstanding capabilities of the individual circuit components. This is however undesirable because it necessitates implementing the whole system in a larger scale whereby additional expenditures are incurred.
As will now be appreciated from the foregoing, the engine control apparatus known heretofore suffers from problems including the generation of electric noise an unacceptably high level, and injury or damage to the distributor 7, the spark plug 8 and the like circuit components due to leakage of abnormally high voltage. In other words, no measures have been adopted for coping with the rise in spark plug demand voltage ascribable to the cut-off of fuel injection in the traction control or ascribable to failure in the failsafe mechanism in the case of the turbo-engine.